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Knocking noise coming from furnace...

I have a furnance located in my attic to control the upstairs of my home. Only when the heat is on, I hear a loud knocking noise coming from the unit. It does not make this noise while running, but can be heard after it shuts off - it may be immediate or several minutes. Sounds like something is turning and rubbing against some metal. Very loud. It does not make this noise when the AC is on. Any thoughts? While sitting in my attic, I narrowed the noise to come from the unit itself. I orignally thought it was the line coming from the heat pump outside vibrating on wood, but that's not the case. Thanks.

The unit is 1 year old. Brand new house. Just wondering if anyone has had a similar issue and can pinpoint the cause since it only happens when the heat is selected and does not occur while running. Exhaust? I've had 2 techs out over the last year, but of course, they never can hear it. But I have the builder coming back this Thursday to troubleshoot.

The unit is 1 year old. Brand new house. Just wondering if anyone has had a similar issue and can pinpoint the cause since it only happens when the heat is selected and does not occur while running. Exhaust? I've had 2 techs out over the last year, but of course, they never can hear it. But I have the builder coming back this Thursday to troubleshoot.

It should be a warranty issue, if it's only a year old. I don't think the builder can help out a lot here. If it's a gas furnace, it could be the heat exchanger expanding and contracting. That's about all I can say in the public forums.

You said it did not make the noise in the a/c mode, most gas furnaces run the a/c and the heat on different speeds, maybe it's doing it in the heat mode because the blower is running at a lower speed, but a service company could probably fix it fairly quick.

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